1. Field of the Invention
This invention relates to a semiconductor device, and more particularly to an improved semiconductor device including a bipolar transistor and a diode which are connected to each other and formed in an isolated area of a semiconductor layer.
2. Description of the Prior Art
Generally, as can be seen in FIG. 1, a bipolar transistor Q and a PN junction diode (or a schottky barrier diode) are often connected in a series for use in, for example, a TTL (transistor transistor logic) circuit. FIG. 1 is a schematic diagram of a semiconductor device consisting of a bipolar transistor and a diode. When an output of the transistor Q is taken out at a point indicated by the arrow Qt.sub.1, the obtained voltage level is the voltage Vce between the collector and emitter of the transistor Q. When an output of the transistor Q is taken out at a point indicated by the arrow Qt.sub.2, the obtained voltage level is the total voltage of the voltage Vce and a forward biased voltage Vf of the diode D.
In order to produce the semiconductor device illustrated in FIG. 1, the bipolar transistor Q and diode D have been formed in each of isolated areas, and then they have been connected with a conductor. Thus, since two isolated areas are required for producing the semiconductor device, it is difficult to produce a semiconductor device of a small size.
In the production of IC or LSI semiconductor devices including the above-mentioned semiconductor device comprised of a bipolar transistor and a diode, there was an attempt made to form the bipolar transistor and diode in one isolated area, whereby the area required for the semiconductor device could be further reduced (for example, it is possible to reduce the required area by 20.about.30% as compared with that for forming the semiconductor device in isolated areas). FIG. 2 is a cross-sectional view of a semiconductor device which is formed in an isolated area and corresponds to the diagram of FIG. 1. In FIG. 2, the reference numerals 1, 2, 3, 4 and 5 represent a p-type silicon semiconductor substrate, an n.sup.+ -type buried layer, an n-type epitaxially grown silicon semiconductor layer, a silicon dioxide insulating film and a surrounding p.sup.+ -type isolation region, respectively. Furthermore, the reference numerals 6, 7, 8 and 9 represent a p.sup.+ -type base region of the bipolar transistor, a p.sup.+ -type region for the diode, an n.sup.+ -type emitter region and an n.sup.+ -type collector contact region, respectively. Thus, the PN junction comprised of the p.sup.+ -type region 7 and the n-type epitaxial semiconductor layer 3 forms a diode, and the base region 6, emitter region 8 and collector region (i.e. a part of the epitaxial semiconductor layer 3) form an NPN transistor in the isolated epitaxial semiconductor layer 3.
However, in such a structure, the region 7, the epitaxial semiconductor layer 3, the base region 6 and the emitter region 8 form a PNPN diode i.e. a thyristor, and it is easy to turn on the PNPN diode by a small current, e.g. a few microamperes in the worst case, namely, a so-called PNPN diode effect occurs.
It is possible to form a schottky barrier diode instead of the above-mentioned PN junction diode. In this case, the semiconductor device comprised of the schottky barrier diode and the above-mentioned transistor functions normally at a small current, e.g. less than 3 mA, but the semiconductor device does not function normally, namely a PNPN diode is turned on by a large current, e.g. more than 3 mA.